In a switching power supply device that controls an output voltage by performing a switching operation using a switching element such as a transistor, variable control of a duty, which is a ratio of an ON-period of the switching element to a switching cycle of the switching element, is performed to control the output voltage so as to be constant.
Devices that generate a PWM signal for performing PWM control are classified into an analog type and a digital type. Examples of a digital type PWM signal generator are described in JP-A-2004-343395 and JP-A-2006-527569.
JP-A-2004-343395 describes a PWM signal generator that uses a ring oscillator including plural differential buffers, which are connected in cascade, to generate a PWM signal with a resolution of a delay time corresponding to one stage of the differential buffers.
JP-A-2006-527569 describes a PWM signal generator that includes plural voltage-controlled buffers connected in series and a delay locked loop (DLL) controlling a delay time of each voltage-controlled buffer so as to be constant.